The present invention relates to a pre-driver, and more particularly, to a pre-driver for connecting a load such as a motor to a drive circuit.
A bridge driver is known in the art as a drive circuit for driving a motor. A half-bridge circuit, which is formed by two NMOS transistors, or a full-bridge circuit (also known as “H-bridge”), which is formed by four NMOS transistors, normally make up the bridge driver. In a half-bridge circuit, two transistors are connected in series between a high potential power supply and ground. Output voltage generated at an output node between the two transistors is supplied as an operational voltage to a load. In a full-bridge circuit, a first set of two series-connected transistors and a second set of two series-connected transistors are connected in parallel between a high potential power supply and ground. A first output voltage, which is generated at a node between the first set of transistors, and a second output voltage, which is generated at a node between the second set of transistors, are supplied to a load. A pre-driver is used as a gate drive circuit that drives the transistors of a bridge driver such as a half bridge circuit or a full bridge circuit.
Japanese Laid-Open Patent Publication No. 2005-354586 describes a pre-driver that drives a high-side transistor (MOS transistor connected to high power supply) arranged in a bridge driver. The pre-driver charges a gate capacitor formed between the source and gate of the high-side transistor with constant current. This prevents the gate drive voltage supplied to the high-side transistor from exceeding the source-gate withstand voltage (gate voltage) of the high-side voltage.
However, in the pre-driver described in the above publication, the rise speed of the gate drive voltage is restricted to be less than or equal to a constant speed to prevent the gate drive voltage from exceeding the gate voltage. This speed restriction effects the responsiveness of the circuit. The responsiveness may be improved by increasing the speed (current amount) used to charge the gate capacitor. However, increasing the current could damage the gate. Thus, to increase the charge current, the circuit scale of the pre-driver would have to be enlarged.